The floating furnace has a series of floaters arranged therein in the direction of transfer, each of which blows gases such as hot air or cold air to float the strip, thereby continuously transferring the strip, contact-free, through the furnace and at the same time heat-treating it with the gas.
The gas pressure of the floaters used in this kind of floating furnace has conventionally been determined by calculating a theoretical gas pressure necessary to float the strip taking into account the thickness, width and specific gravity (kind of steel) (in this specification, it is also called shape conditions) of the strip and then air volume of floater blowers has been controlled so that the floaters in the furnace can maintain that gas pressure.
In the floating furnace that performs such a control, not only is the calculation, based on a variety of conditions such as thickness, width and specific gravity, complicated but also there is a possibility of artificial errors that during the calculating process these conditions may be incorrectly measured or the resultant figures erroneously entered. These artificial errors will make the floating of a strip inappropriate, leading to damages to the strip due to contact with members in the furnace. This causes lowering of the reliability that the strip can be heat-treated without a damage.
Another problem is that every time a metal strip to be heat-treated is changed in shape condition, a theoretical pressure necessary for floating needs to be calculated and the resultant value preset, rendering the control even more complex.
Further, when metal strips with greatly differing shape conditions are connected for continuous heat treatment in the furnace, the conventional floating furnace has the following problem. When the boundary of the two strips passes through the furnace, if the gas pressure of the in-furnace floaters are set to that used for the preceding strip, the front end portion of the succeeding strip floats too high or too low. Conversely, when the gas pressure is set for the succeeding strip, the floating height of the rear end portion of the preceding strip becomes too low or too high. As a result, the front or rear end portion of the strips may contact the members in the furnace and be damaged.